Optical Absorption Spectrum ofNd3+-Doped LaF3Single Crystal: The Evidence of a Hidden Selection Rule

The symmetry of the ${\mathrm{La}}^{+3\mathrm{}}$ site in La${\mathrm{F}}_3$ has been determined by x-ray and paramagnetic resonance. The x-ray results show a $C_{2v}$ site symmetry in agreement with paramagnetic resonance. The optical absorption spectrum shows conclusive evidence of a selection rule contrary to the fact that a $C_{2v}$ site symmetry gives no selection rule. Hence, there exists a hidden selection rule.